1. Field of the Invention
The present invention relates to a high-temperature sulfidation-corrosion resistant nickel-base alloy for use as, for example, a material for a gas expander turbine which facilitates utilization of recovered energy from exhaust gas discharged from a fluidized-bed catalytic cracking apparatus of a petroleum refining plant.
2. Description of the Prior Art
Heat-resistant nickel-base alloys which have been used as a material for a turbine rotor which is exposed to high temperatures have a high resistance against oxidation and creep as well as a high temperature strength.
Many heat-resistant nickel-base alloys contain small amounts of titanium (Ti) and aluminum (Al) to precipitate a gamma prime (.gamma.') phase of Ni.sub.3 (Ti,Al) for achieving good high-temperature strengths. Usually, those heat-resistant nickel-base alloys contain less than 1.6 weight % of aluminum and more than 2.5 weight % of titanium. As the total amount of added titanium and aluminum in a heat-resistant nickel-base alloy increases, the forgeability of the alloy decreases. If it is necessary that the total amount of titanium and aluminum exceeds 6 weight %, then the alloy will more often be formed as castings than forgings.
High-temperature mechanical devices such as turbines, boilers, etc. that find use in combustion gas atmospheres are known to be subject to a process of "hot corrosion" in which a molten salt containing sodium (Na), sulfate (SO.sub.4), vanadium (V), and/or chlorine (Cl) play a certain role. It has also been reported that nickel-base alloys suffer a catastrophic sulfidation corrosion at a temperature of or higher than 700.degree. C. due to a direct reaction between gases and metals without mediation of a molten salt. The catastrophic sulfidation corrosion is believed to occur to nickel-base alloys due to, among other causes, the formation of a eutectic of Ni--Ni.sub.3 S.sub.2 which has a low melting point of 645.degree. C.
There have recently been developed energy recovery systems for recovering the energy of exhaust gases which are discharged from fluidized-bed catalytic cracking apparatus in order to save energy in petroleum refining plants. As a material for manufacturing turbine blades of the gas expander for use in such an energy recovery system, "Waspaloy (trade name)" was experimentally employed which is a typical heat-resistant nickel-base alloy. However, a sulfidation corrosion has been found at proximal ends of the turbine blades which were used in a temperature range lower than usual temperatures for causing a sulfidation corrosion resulting in an undue reduction in the service life of the turbine blades. An inspection of a cross section of the corroded region has indicated that, as shown in FIG. 1 of the accompanying drawings, the corroded region has developed an upper layer including nickel sulfide and a lower layer including chromium sulfide, and the sulfidation has been in progress deeply along alloy grain boundaries. However, the inspection of the corroded region has not revealed any products including Na, Cl, SO.sub.4, and/or V which would otherwise give a sign of the formation of a molten salt.
FIG. 2 of the accompanying drawings is a microscopic structural representation showing, in cross section, results of a high-temperature sulfidation-corrosion test conducted on a conventional nickel-base alloy in a sulfidizing gas atmosphere for the purpose of finding causes of the sulfidation corrosion. The high-temperature sulfidation-corrosion test was carried out under a sulfur partial pressure (PS.sub.2) of 10.sup.-8.6 atm. at a temperature of 600.degree. C. for 96 hours. In the high-temperature sulfidation-corrosion test, the nickel-base alloy suffered a sulfidation corrosion as shown in FIG. 2, which was a reproduction of the corroded region shown in FIG. 1. It was confirmed as a result of the high-temperature sulfidation-corrosion test that a grain boundary sulfidation corrosion is caused by a direct reaction between the metals and the gases without the formation of a molten salt containing Na, Cl, SO.sub.4, and/or V. There have been almost no reports on the occurrence of a grain boundary sulfidation corrosion on nickel-base alloys in a sulfidizing gas atmosphere at a temperature of or lower than 645.degree. C. Consequently, any behaviors and mechanisms of such a grain boundary sulfidation corrosion have not been clarified in the art so far.